The present invention relates to a method for monitoring and controlling vehicular hydraulic brake slip control devices supplied with energy from an external source on the occurrence of failures in the hydraulic system, in particular on the loss of hydraulic fluid or on a pressure drop resulting from defects. The devices concerned are of the type in which the pressure is transmitted to the brake wheel cylinders via one or several static brake circuits and/or via one or several dynamic circuits, preferably brake circuits, wherein in the event of a control action taking place, i.e., when the brake slip control responds, fluid flow is admitted from the dynamic circuit into the static brake circuits, with reservoirs supplying the hydraulic system with the hydraulic fluid. Apparatus to implement this method are also part of this invention.
In brake slip control devices of the prior art (German Pat. DE-OS Nos. 3,040,561 and 3,040,562), valves are provided which, in the event of a pressure loss in the external energy system, will be hydraulically switched to a closed position, thereby inhibiting the dynamic fluid flow into the static brake circuits. Further, as a result of the pressure failure, a valve in the common pressure fluid return line from the pressure-reducing valves associated with the control units will also be closed by hydraulic means. This prevents the static circuits from being controlled to an empty condition on the failure of external energy. As a result controlling the brake slip is no longer possible in the event of a disturbance. In this known device, a drop in the reservoir fluid level resulting from a leak will invariably not activate the measures described until, as a result of this loss of fluid, the pressure in the system drops below the switch-over value of the hydraulically activated valves. Further, it is known to provide for deactivation of the entire brake slip control on the occurrence of any disturbance for safety reasons to avoid any deterioration, at least when compared to devices having no brake slip control system. This has, however, the disadvantage that wheel lock with all the known adverse effects may occur as early as on a failure of one brake circuit or even as a result of a relatively insignificant fault.